The mechanism of protein absorption in the proximal tubule will be examined. The following questions will be addressed: 1. What is the effect of acidification of proximal tubule fluid on the absorption of proteins? The uptake of anionic, neutral and cationic proteins in a normal (7.4) and an acidic (6.8) pH will be measured and compared. 2. Is there an axial heterogeneity in the uptake of proteins along the length of the proximal tubule and a selectivity in the uptake of proteins based on charge or the number of cationic amino acid residues, arginine and lysine, of the protein? We will determine and compare the uptake of proteins by various segments of the proximal tubule and test for selectivity of protein uptake by competition experiments among proteins of similar and dissimilar charge or between arginine-rich proteins, lysine-rich proteins or arginine-rich and lysine-rich proteins. 3. Is there a role for calcium in protein handling by the proximal tubule? If so, what is the mechanism of its action in the uptake of proteins by the proximal tubule? Protein uptake will be measured at varied levels of intracellular and extracellular calcium, and in the presence of calcium channel blockers or cAMP. We will also examine the effects of the inhibitors of certain cytoskeletal structures including microtubules and microfilaments, and the calcium binding protein, calmodulin, on the uptake of protein. 4. What is the effect of conformational changes and chemical modification of amino acid residues of protein on the uptake of protein by the proximal tubule? We will examine the effects of conformational changes of albumin by fatty acid binding on albumin uptake by the proximal tubule and the effects of specific chemical modification of cationic residues, lysine and arginine, on the protein uptake. The protein uptake will be determined in the isolated perfused proximal tubule of rabbit kidney using proteins labeled with 3H by reductive methylation.